1. Technical Field
The present invention relates generally to high reliability storage and retrieval of data in a storage device having a spindle motor and more particularly to a method and apparatus for assuring that data read and write operations do not coincide with commutation of the spindle motor poles for enhanced data readback reliability.
2. Description of the Background Art
Magnetic disk drives utilized for the storage of information are well-known. Data is stored in the form of magnetic transitions created on the surfaces of one or more magnetically coated storage disks stacked vertically on a rotatable spindle motor assembly. The data is organized into a plurality of annular rings or tracks having a desired track format. The specific format of a data track depends upon the particular design of the disk drive system, but will invariably include a number of non-user-data fields. Data is stored and retrieved on a disk surface using a head assembly, movement of the assembly relative to the storage medium being controlled by a servo system.
One or more disks are mounted to a spindle motor through a rotatable hub. Today's disk drives predominantly use brushless DC spindle-motors having a fixed number of poles and windings. A commutation voltage waveform must be applied to each winding in a specific sequence to cause rotation of a desired speed and direction. The timing of the switches is preferably optimized to reduce power consumption. An example of a brushless DC spindle motor is provided in DC Motors Speed Controls Servo Systems: An Engineering Handbook, 5th Ed., August, 1980, Chapter 6, published by Electro-Craft Corporation. 1600 Second Street South, Hopkins, Minn. 55343. Switching of the pole commutation currents according to the predefined sequence is controlled by a spindle motor controller.
Switching points will frequently coincide with read and/or write operations. Unfortunately, switching of the commutation voltages produces noise spikes which may interfere with the channel and the read/write transducers, adversely impacting data reliability.
One approach for avoiding noise during data transfers is disclosed in Unexamined Japanese Patent Publication No. 58-171766, wherein the data surfaces of a storage device are formatted such that each switching point corresponds to a gap in one of the data fields. Thus switching only occurs when a gap is passing beneath the active head. This method requires a same, fixed number of data sectors per track, the number of sectors being a multiple of the number of switches required to commutate the motor, to assure that a gap will coincide with current switching. But data sectors today typically do not include gaps of sufficient length to accommodate commutation switching. Moreover, data is no longer recorded at a fixed frequency, instead being recorded in bands of different data frequencies for increased data capacity. Consequently, each band will have a different number of data sectors per track. In addition, the formats of data sectors in disk drives employing sectored servo architectures will also vary, some of the sectors of a track being interrupted or "split" by servo information. This means that the placement of gaps will also vary. Finally, today's disk drives skew the information recorded on one disk surface relative to the next adjacent surface for performance reasons. The Japanese reference does not account for such skewing in spindle motor commutation.
What is needed, therefore, is a method and apparatus for use in today's disk drive designs for controlling spindle motor current switching such that switching does not occur during a read or write operation, particularly in disk drives implementing banded recording, sector skewing, and other storage disk formats having irregularly placed nondata fields.